1. Field of the Invention
The present invention relates to an optical fiber applicable to an optical transmission line for optical communication systems.
2. Background of the Invention
Single-mode optical fibers have conventionally been used as optical transmission lines in optical communications. Such single-mode optical fibers have a zero-dispersion wavelength in the vicinity of a wavelength of 1.3 xcexcm, a positive dispersion slope in the 1.55-xcexcm wavelength band, and a dispersion of about 18 ps/nm/km at a wavelength of 1.55 xcexcm.
Single-mode optical fibers having optical characteristics such as those mentioned above are defined in G652 and G654 standards of ITU-T, and have a simple refractive index profile composed of a core and a cladding. The 1.55-xcexcm wavelength band (1500 nm to 1600 nm) is applied to a signal wavelength band since silica glass, which is the main ingredient of optical fibers, has a low attenuation in this wavelength band. On the other hand, as mentioned above, a single-mode optical fiber has a positive dispersion in the 1.55-xcexcm wavelength band. Hence, in order to compensate for this positive dispersion, an example constructing an optical communication system by combining a dispersion-compensating optical fiber having a negative dispersion with a large absolute value in the 1.55-xcexcm wavelength band and the single-mode optical fiber is reported in M. Murakami, et al., EOCC""98, pp. 313-314 (1998), for instance.
The inventors have studied conventional optical fibers and, as a result, have found a problem as follows. Namely, the single-mode optical fibers defined in the above-mentioned G652 and G654 standards have an effective area which is greater than that of dispersion-compensating optical fibers and the like, and is about 80 xcexcm2 at 1550 nm. Therefore, the single-mode optical fibers are relatively effective in reducing nonlinear optical phenomena.
Meanwhile, for elongating repeater intervals in an optical communication system, optical signals incident thereon are required to increase their power. Here, optical fibers utilized in optical transmission lines between repeaters must further increase their effective area, so as to fully restrain nonlinear optical phenomena from occurring even when optical signals having a high power propagate through the optical fibers.
However, the optical fibers defined in G652 and G654 standards cannot fully suppress the occurrence of nonlinear optical phenomena. Therefore, it has been difficult to carry out optical communications over a longer distance by utilizing the conventional optical fibers.
For overcoming the problem such as that mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure suitable for long-distance optical communications, and an optical communication system including the same.
The optical fiber in accordance with the present invention is an optical waveguide which is mainly composed of silica glass and is disposed in at least one of areas between an optical transmitter for outputting an optical signal and an optical receiver for receiving the optical signal, between the optical transmitter and a repeater including an optical amplifier or the like, between repeaters, and between a repeater and the optical receiver. Applicable to this optical fiber is any of an optical fiber having a matched type refractive index profile obtained when the cladding region surrounding the outer periphery of the core region is constituted by a single layer, and an optical fiber having a depressed cladding type refractive index profile obtained when the cladding region is constituted by at least an inner cladding in contact with the core region and an outer cladding having a refractive index higher than that of the inner cladding.
This optical fiber has, as characteristics at a wavelength of 1.55 xcexcm (1550 nm), an effective area of at least 110 xcexcm2, a dispersion of 18 to 23 ps/nm/km, and a dispersion slope of 0.058 to 0.066 ps/nm2/km, whether it has the above-mentioned matched type refractive index profile or depressed cladding type refractive index profile.
In particular, it is preferred in the optical fiber having a matched type refractive index profile that the relative refractive index difference of the core region with respect to the cladding region be +0.15% to+0.30%. Obtained in this case is an optical fiber having a cutoff wavelength of 1.3 xcexcm to 1.75 xcexcm, and an effective area of at least 110 xcexcm2 at a wavelength of 1.55 xcexcm.
On the other hand, the optical fiber having a depressed cladding type refractive index profile comprises a core region, an inner cladding region disposed at the outer periphery of the core region, and an outer cladding disposed so as to surround the outer periphery of the inner cladding, and has an effective area of at least 110 xcexcm2 at a wavelength of 1.55 xcexcm. Here, the inner cladding and outer cladding constitute a cladding region surrounding the outer periphery of the core region, the inner cladding has a refractive index lower than that of the core region, and the outer cladding has a refractive index higher than that of the inner cladding.
In any of the optical fibers having the above-mentioned refractive index profiles, the effective area is preferably at least 120 xcexcm2, more preferably 150 xcexcm2, at a wavelength of 1.55 xcexcm. Enlarging the effective area as such effectively restrains nonlinear optical phenomena from occurring even when the power of incident optical signal (1.55-xcexcm wavelength band) is enhanced, thereby enabling optical communications over a longer distance.
Preferably, this optical fiber has a transmission loss of 0.30 dB/km or less at a wavelength of 1.38 xcexcm (1380 nm). Further preferably, the cutoff wavelength (the cutoff wavelength of LP11 mode measured in a state where an optical fiber having a length of 2 m is loosely wound about a mandrel having a radius of 140 mm by one turn) is 1.3 xcexcm to 1.75 xcexcm. In this case, a single mode is assured, in a cable having over 1 km length, in the 1.55-xcexcm wavelength band, and also the bending loss is restrained from increasing (which is advantageous for cabling). For realizing long-distance optical communications, it is preferred that the transmission loss at a wavelength of 1.55 xcexcm be 0.180 dB/km or less at most.
For satisfying the condition concerning cutoff wavelength mentioned above, the core region preferably has an outside diameter of 11.5 xcexcm to 23.0 xcexcm. If the outside diameter (fiber diameter) of the cladding region is set to 130 xcexcm to 200 xcexcm, then microbend loss can be reduced, and the probability of breakage can be lowered.
In the optical fiber having a depressed cladding type refractive index profile, the ratio 2b/2a of the outside diameter 2b of the inner cladding to the outside diameter 2a of the core region is preferably 1.1 to 7. This is because of the fact that the cutoff wavelength can be shortened without increasing the bending loss and that the effective area can be enlarged while in a state where the single mode is assured in the 1.55-xcexcm wavelength band even if the outside diameter of the core region is enlarged. Preferably, the refractive index differences of the core region and inner cladding with respect to the outer cladding are +0.15% to +0.50% and xe2x88x920.15% to xe2x88x920.01%, respectively. Under such a condition, an optical fiber having a cutoff wavelength of 1.3 xcexcm to 1.75 xcexcm and an effective area of at least 110 xcexcm2 at a wavelength of 1.55 xcexcm is obtained.
Preferably, in the optical fiber in accordance with the present invention, the core region is made of silica glass which is not intentionally doped with impurities (hereinafter referred to as pure silica glass), whereas the cladding region (composed of the inner and outer claddings in the case of the optical fiber having a depressed cladding type refractive index profile) is made of silica glass doped with fluorine. In such a configuration, since the core region is not intentionally doped with impurities such as Ge element, the transmission loss can be suppressed by about 0.02 dB/km as compared with optical fibers whose core region is doped with Ge. In such a configuration in which only the refractive index of the cladding region is controlled with reference to the core region, however, the amount of impurities added to the cladding region must be enhanced in order to enlarge the difference in refractive index between the core region and cladding region. If the core region is doped with chlorine which yields a smaller increase of transmission loss upon doping, as compared with Ge, Al, and P, so as to enhance the refractive index of the core region with respect to pure silica glass, then a sufficient refractive index difference can be generated between the core region and cladding region even when the amount of fluorine added to the cladding region is lowered. Namely, the amount of addition of fluorine, which causes the transmission loss to increase, can be lowered without affecting optical characteristics.
The optical fiber in accordance with the present invention, in the core region in particular, may have a refractive index profile which gradually changes from a center part of the core region toward an outer peripheral part thereof. Specifically, a radial refractive index profile form in the core region is controlled such that, in a cross section of the core region, the refractive index difference xcex94na(r) at a location radially separated by a distance r (0xe2x89xa6rxe2x89xa6xcex1) from the center part of the core region with respect to a reference region of the cladding region is approximated by the following expression:
xcex94nxcex1(r)=xcex94nxcex1(0)xc2x7|1xe2x88x92(r/xcex1)xcex1|xe2x80x83xe2x80x83(1)
where
xcex94na(0) is the relative refractive index difference of the center part of the core region with respect to the reference region of the cladding region; and
xcex1 is a real number of 1 to 10.
The refractive index profile whose part corresponding to the core region is expressed by the above-mentioned approximate expression (1) attains a dome-shaped form in which a center portion is raised from a peripheral portion in the part corresponding to the core region.
Also, the radial refractive index profile form in the core region may be controlled such that, in a cross section of the core region, the refractive index difference xcex94na(r) at a location radially separated by a distance r (0xe2x89xa6rxe2x89xa6xcex1) from the center part of the core region with respect to a reference region of the cladding region is approximated by the following expression:
xcex94nxcex1(r)=xcex94nxcex1(xcex1)xc2x7|1xe2x88x92xcex3xc2x7(1xe2x88x92r/xcex1)xcex2|xe2x80x83xe2x80x83(2)
where
xcex94na(a) is the relative refractive index difference at a location corresponding to the outer periphery of the core region with respect to the reference region of the cladding region;
xcex2 is a real number of 1 to 10; and
xcex3 is a positive real number.
The refractive index profile whose part corresponding to the core region is expressed by the above-mentioned approximate expression (2) attains a form in which a peripheral portion is raised from a center portion in the part corresponding to the core region. In any of the cases with the above-mentioned approximate expressions (1) and (2), the relative refractive index difference xcex94na in the core region is set with reference to the location yielding the lowest refractive index. As a consequence, the reference region of the cladding region corresponds to the single cladding region itself in the case of the optical fiber having a matched type refractive index profile, and the inner cladding in the case of the optical fiber having a depressed cladding type refractive index profile.
The optical fibers having the above-mentioned structures are applicable to optical communication systems propagating optical signals in a wavelength band of 1.35 to 1.52 xcexcm in addition to the 1.55-xcexcm wavelength band of 1530 to 1565 nm and 1.58-xcexcm wavelength band of 1570 to 1620 nm. Also, such an optical communication system may comprise an optical amplifier, disposed upstream the optical fiber, for amplifying a plurality of wavelengths of optical signals. Such an optical amplifier may include an erbium-doped fiber amplifier comprising an amplification optical fiber doped with erbium, and a Raman amplifier.
Here, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area Aeff is given by the following expression (3):
                              A          eff                =                  2          ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    r                                                              )                                                          (        3        )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region. On the other hand, the dispersion slope in this specification is given by the gradient of the graph indicating the wavelength dependence of dispersion.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.